This invention relates to an improved method of preparing composite superconducting wire. More specifically, this invention relates to an improved method for preparing a composite superconducting wire consisting of multifilaments of Nb.sub.3 Sn in a copper matrix.
One method presently used for preparing multifilament superconducting wire such as Nb.sub.3 Sn in a predominantly copper matrix consists of drilling a plurality of evenly spaced holes in a copper or bronze billet, inserting niobium rods in each hole, and extruding and drawing the billet in several steps until the niobium rods are reduced to the desired filament size. The wire is then coated with tin and heated to react the tin and niobium in order to form Nb.sub.3 Sn. The process is expensive and exacting and the size generally limited to filaments larger than 2 .mu.m in diameter.
In another method, cylindrical rods of niobium are inserted into tubes of copper to form composite rods. A large number of the composite rods, which are hexagonal in cross section to improve packing density, are tightly packed into an extrusion can of normal metal, sealed and reduced in cross section by various methods of hot and cold working to produce a multifilament composite wire. The wire then must be coated with tin or heated in a tin-containing atmosphere to form Nb.sub.3 Sn. Preparation of the composite rods in difficult and time-consuming because of the close tolerances necessary to ensure a good bonding between the metals and to prevent contamination. The hexagonal outer surface of the tubes must be dimensionally accurate so that large numbers of the tubes can be tightly packed into the extrusion can to prevent trapping of gas or other contaminants between the rods which would affect superconductivity. Thus preparation of superconducting wire by this method is time-consuming and expensive.
To overcome the difficulties of the prior art methods for preparing composite multifilament superconductors, a ductile alloy suitable for preparing composite superconductors was developed. The alloy consists of copper containing at least 15 weight percent niobium which is present in the copper as discrete, randomly distributed and oriented, elongated discontinuous dendritic-shaped particles having an aspect ratio of about 50 to 100, the dendrites generally being from about 1 to 25 .mu.m in diameter and from about 100 to 250 .mu.m in length. The alloy, a method of making the alloy and a method of preparing composite superconducting wire from the alloy are described in copending U.S. patent application Ser. No. 19,808, filed Mar. 12, 1979, now U.S. Pat. No. 4,378,330, dated Mar. 29, 1983 assigned to the common assignee, and incorporated herein by reference. As described therein, when a billet of the alloy is mechanically reduced to wire, the niobium dendrites elongate and align parallel to the axis of the wire to give long discontinuous filaments of niobium in a copper matrix. The wire is then coated with tin and heated to diffuse the tin into the wire to react with the niobium to form Nb.sub.3 Sn which is a brittle compound. It was found that the tin will diffuse at temperatures sufficiently low to permit coils for magnets or similar devices to be wound from the ductile tin-coated wire before heating to diffusion temperature without destroying insulation used in preparing the coils.
While these methods will produce composite wire having good superconducting and mechanical properties, the fabrication procedures are not completely satisfactory for the preparation of large quantities of superconducting wire. For example, the application of a layer of tin to long lengths of small diameter wire is expensive. Thick layers of tin applied to the outer surface of large diameter wires are unstable and tend to "ball-up" when they are first melted in the reaction process. This produces brittleness in the tin-rich regions and poor superconducting performance in the tin-deficient regions. Braiding the cable wire after tin plating, but before the diffusion, tends to smear the relatively soft tin layer, while diffusion times for large cross-sectional wire are rather long, making the process time-consuming and increasing the cost.